1. Field of the Invention
The present invention relates to a plasma display device, and in particular, it relates to a plasma display device driven in a subframe mode.
2. Description of the Related Art
A plasma display panel (referred to as PDP, below) is a kind of flat display widely used, for example, in various OA apparatus and TVs, since the panel structure thereof is very simple and all the elements of the panel structure can be made using a thick film printing technique.
A conventional color PDP of a triple-electrode type is structured with two glass plates arranged in parallel with each other to form a discharge space. On one of the glass plates, address electrodes and a phosphor are provided while X electrodes and Y electrodes are provided on another glass plate to intersect each other at right angles. A so-called "subframe mode" is known to drive such a PDP of the triple-electrode type. In this driving mode, one frame is divided into, for example, 8 subframes, each of which has a sustaining discharge period. The respective sustaining discharge periods of the subframes are set to a ratio of 1:2:4:16:32:64:128 (although the ratio is constant in this example, there is no need for it to be always constant), and these subframes are combined to realize a grey-scale display.
In such a plasma display device driven in the subframe mode, the brightness of the PDP is determined by the total number of sustaining pulses applied to the PDP during one frame. That is, it is determined by the total number of sustaining pulses in all the subframes of one frame. In actuality, as the number of sustaining pulses applied to electrodes during one frame increases, the brightness of the display increases. Therefore, to display a bright image on the plasma display device, a large number of sustaining pulses are necessary during one frame while a small number of sustaining pulses are enough to display an image having a relatively low brightness.
PDPs are usually driven by image signals supplied from an external device, such as a TV tuner and a personal computer, connected to the PDPs. The driving frequencies of these external devices are not the same as each other. Since the length of one frame of a PDP is determined depending on one period of a driving signal, that is, a vertical synchronizing signal, introduced from an external device, the real length of one frame of the PDP varies depending on what kind of external device is connected with the plasma display device.
According to the variation of the frame length as mentioned above, a deficiency arises as follows. When the frame length becomes shorter than that expected for the plasma display device, the length of one driving period of the plasma display device required to display one frame exceeds the real frame length when displaying a very bright image with a large number of sustaining pulses. This results in an extraordinary display on the plasma display device.
On the other hand, when the frame length becomes much longer than that expected in advance for the plasma display device, the length of one driving period of the plasma display device required to display one frame becomes shorter than the expected value. This results in the unnecessary extension of a quiescent period in one frame, thus lowering the brightness of the display.
As mentioned above, the prior art plasma display device driven in a subframe mode is disadvantageous in that it does not have enough flexibility to accommodate various types of external devices having different driving frequencies.